decidua (de-sid'u-ah)

An adventitious envelope formed around the fertilized ovum on its entrance into the uterine cavity, consisting of the mucous membrane lining the body of the uterus in a state of heightened development and thickening; so called because it is eventually cast off. d. protrusa, d. reflex a. That portion of the d. which, being continuous with the d. vera, covers the ovum as it projects into the uterine cavity, and finally, coming to lie in contact with the d. vera, constitutes the inner layer of the decidual sac taken as a whole, d. serotina. The layer or layers of decidual substance which intervene between the developed placenta and the uterine wall. d. tunica uteri, d. uteri, d. uterina, d. vera. That portion of the mucous membrane lining the cavity of the uterus, which, when the fecundated ovum comes to be imbedded in the membrane, continues to line the general cavity of the uterus in every part except that occupied by the attachment of the ovum, placental d., placental d. serotina. That layer of the d. serotina which covers the cotyledons, uterine d. serotina. The outer portion of the d. serotina. [Lat., deciduus, from de, from, + cadere, to fall.]